The present invention relates to a method of making improved polyolefinic elastic articles from cured, irradiated or crosslinked amorphous ethylene interpolymers. In particular, the invention relates to a method of making a shaped article (e.g. film or fiber) characterized by improved elevated temperature elasticity as well as washability and dry ability. The inventive elastic article comprises a substantially cured, irradiated, or crosslinked (or curable, irradiated or crosslinkable) homogeneously branched ethylene interpolymer characterized as having a density less than 0.90 g/cm3 and containing at least one nitrogen-containing stabilizer. The improved elastic article of the present invention is particularly suitable for use in applications where good elasticity must be maintained at elevated temperatures and after laundering such as, for example, elastic waist bands of undergarments and other clothing.
Materials with excellent stretchability and elasticity are needed to manufacture a variety of disposal and durable articles such as, for example, incontinence pads, disposable diapers, training pants, clothing, undergarments, sports apparel, automotive trim, weather-stripping, gaskets, and furniture upholstery. Stretchability and elasticity are performance attributes which can, for example, function to effectuate a closely conforming fit to the body of the wearer or to the frame of the item. While numerous materials are known to exhibit excellent stress-strain properties and elasticity at room temperatures, it is often desirable for elastic materials to provide a conforming or secure fit during repeated use, extensions and retractions at elevated temperatures such as at body temperatures or in automobile interiors during summer months. Maintaining tight tolerances throughout temperature cycles are also instances where elevated temperature elasticity is important. Further, where an elastics material is employed in clothing or garment items, the material must maintain its integrity and elastic performance after laundering.
Disposable elastic articles are typically elastic composite materials prepared from a combination of polymer film, fibers, sheets and/or absorbent materials as well as a combination of fabrication technologies. Whereas elastic fibers can be prepared by well known processes such as spun bonding, melt blowing, melt spinning and continuous filament wounding techniques, the film and sheet forming processes typically involve known extrusion and coextrusion techniques, e.g., blown film, cast film, profile extrusion, injection molding, extrusion coating, and extrusion sheeting.
Conversely, durable elastic articles are often molded or profile items such as, for example, automotive door and window trim, clothing waist band threads or strips, and building weather-stripping. Such durable articles can be made by well known molding, thermoforming and profile technologies.
A material is typically characterized as elastic when it is characterized as having a high percent elastic recovery (i.e., a low percent permanent set) after application of a biasing force. Ideally, elastic materials are characterized by a combination of three, temperature independent properties, i.e., a low percent permanent set, a low stress or load at strain, and a low percent stress or load relaxation. That is, there should be at low to elevated service temperatures (1) a low stress or load requirement to stretch the material, (2) no or low relaxing of the stress or unloading once the material is stretched, and (3) complete or high recovery to original dimensions after the stretching, biasing or straining is discontinued.
Lycra(copyright) is the trademark of Dupont Fibers for its elastic spandex fibers. The U.S. International Trade Commission defines spandex fiber as a manufactured fiber in which the fiber-forming substance is a long-chain synthetic polymer comprised of at least 85 percent segmented polyurethane. Lycra is known to exhibit nearly ideal, temperature independent elastic properties rendering it very suitable for use in garments, sports apparel and swimsuits. However, one significant shortcoming of Lycra is it typically exhibits fair to poor elevated temperature serviceability and washability. Similar to ordinary uncrosslinked polyolefin-based elastic materials, Lycra articles tend to lose their integrity and shape and elastic properties when subjected to elevated service temperatures such as during laundering and drying. Another major shortcoming of Lycra is its cost. That is, Lycra tends to be extremely cost prohibitive for a many of applications.
Elastic materials such as films, strips, coating, ribbons and sheet comprising at least one substantially linear ethylene polymer are disclosed in U.S. Pat. No. 5,472,775 to Obijeski et al., the disclosure of which is incorporated herein by reference. However, U.S. Pat. No. 5,472,775 does not disclose the performance of these materials at elevated temperatures (i.e., at temperatures above room temperature), nor their performance after laundering.
WO 94/25647, the disclosure of which is incorporated herein by reference, discloses elastic fibers and fabrics made from homogeneously branched substantially linear ethylene polymers. The fibers are said to posses at least 50 percent recovery (i.e., less than or equal 50 percent permanent set) at 100 percent strain. However, there is no disclosure in WO 94/25647 regarding the elasticity of these fibers at elevated temperatures or the effects of laundering on these fibers.
WO 95/29197, the disclosure of which is incorporated herein by reference, discloses curable, silane-grafted substantially ethylene polymers which are useful in wire and cable coatings, weather-stripping, and fibers. In the Examples, inventive samples include fibers comprising silane-grafted substantially ethylene polymers having densities of 0.868 g/cm3 and 0.870 g/cm3. The inventive examples are shown to exhibit improved elastic recovery at elevated temperatures. However, there is no disclosure in WO 95/29197 regarding the percent stress or load relaxation performance at elevated temperatures for these silane-crosslinked fibers, nor is there any disclosure as to washability.
U.S. Pat. No. 5,324,576, the disclosure of which is incorporated herein by reference, discloses an elastic nonwoven web of microfibers of radiation crosslinked ethylene/alpha olefin copolymers, preferably having a density less than 0.9 g/cm3. In the examples set forth in U.S. Pat. No. 5,324,576, ethylene polymers having polymer densities greater than or equal to 0.871 g/cm3 are subjected to electron beam radiation. However, there is no disclosure regarding the elastic performance of these radiated polymers at elevated temperatures, nor is there any disclosure regarding their resistance to washing and drying.
U.S. Pat. No. 5,525,257 to Kurtz et al., the disclosure of which is incorporated herein by reference, discloses that low levels of irradiation of less than 2 megarads of Ziegler catalyzed linear low density ethylene polymer results in improved stretchability and bubble stability without measurable gelation. However, ""257 provides no disclosure respecting the elasticity and/or washability at elevated temperatures.
U.S. Pat. No. 4,957,790 to Warren, the disclosure of which is incorporated herein by reference, discloses the use of pro-rad compounds and irradiation to prepare heat-shrinkable linear low density polyethylene films having an increased orientation rate during fabrication. In the examples provided therein, Warren employs Ziegler catalyzed ethylene polymers having densities greater than or equal to 0.905 g/cm3.
Various compounds are disclosed in the art and/or sold commercially as high temperature stabilizers and antioxidants. However, the criteria employed to distinguish these compounds as stabilizers and antioxidants typically relates to their ability to resistance yellowing, crosslinking and/or the ill-effects of irradiation (e.g., gamma irradiation for purposes of sterilization).
In other instances, different types of stabilizers are equated to one another or are said to perform comparably. For example, it is known that hindered phenolic stabilizers (e.g., Irganox(copyright) 1010 supplied by Ciba-Geigy) can be as effective as hindered amine stabilizers (e.g., Chimassorb(copyright) 944 supplied by Ciba-Geigy), and vice versa. In a product brochure entitled, xe2x80x9cChimassorb 944FL: Hindered Amine Light Stabilizer Use and Handlingxe2x80x9d, printed 1996, Ciba-Geigy states Chimassorb 9944 xe2x80x9cgives long-term heat stability to polyolefins by a radical trapping mechanism similar to that of hindered phenols.xe2x80x9d
Further, there is some belief that there is no universally effective stabilizer for polymers as the definition for stability inevitably varies with each application. In particular, there is no effective stabilizer for washable, high temperature serviceable polyolefinic elastic materials.
In general, stabilizers are known to inhibit crosslinking. In regard to crosslinking generally, there are several disclosures relating to radiation resistant (e.g., gamma and electron beam) polymer compositions comprising amine stabilizers. Such disclosures typically teach relatively high levels of amine stabilizer (for example, greater than or equal to 0.34 weight percent) are required where inhibition of crosslinking, discoloration and other undesirable irradiation effects is desired. Another examples include stabilized disposal nonwoven fabrics (see, e.g., U.S. Pat. No.5,200,443, the disclosure of which is incorporated herein by reference) and stabilized molding materials (e.g. syringes). Gamma sterilization resistant fibers, including amine coatings and the use of hybrid phenolic/amine stabilizers are also known. See, e.g., U.S. Pat. No. 5,122,593 to Jennings et al., the disclosure of which is incorporated herein by reference.
Stabilized polyethylene compositions with improved resistance to oxidation and improved radiation efficiency are also known. M. Iring et al. in xe2x80x9cThe Effect of the Processing Steps on the Oxidative Stability of Polyethylene Tubing Crosslinked by Irradiationxe2x80x9d, Die Angew. Makromol. Chemie, Vol. 247, pp. 225-238 (1997), the disclosure of which is incorporated herein by reference, teach that amine stabilizers are more effective towards inhibiting electron-beam irradiation effects (i.e., provide better resistance against oxidation) than hindered phenols.
WO 92/19993 and U.S. Pat. No. 5,283,101, the disclosures of which are incorporated herein by reference, discloses launderable retroreflective appliquxc3xa9s comprised of a multicomponent binder composition consisting of an electron-beam curable elastomer, crosslinker(s), and coupling agent(s) and optional colorants, stabilizers, flame retardants and flow modifiers. The allegedly inventive appliquxc3xa9s are said to be capable of withstanding ordinary household washing conditions as well as more stringent industrial washings without loss of retroreflectiveness. Illustrative examples of electron-beam curable elastomers of the binder are said to be xe2x80x9cchlorosulfonated polyethylenes, ethylene copolymers comprising at least about 70 weight percent of polyethylene such as ethylene/vinyl acetate, ethylene/acrylate, and ethylene/acrylic acid, and poly(ethylene-co-propylene-co-diene) (xe2x80x9cEPDMxe2x80x9d) polymers.xe2x80x9d Optional stabilizers are described to be xe2x80x9cthermal stabilizers and antioxidants such as hindered phenols and light stabilizers such as hindered amines or ultraviolet stabilizersxe2x80x9d. Although there is an equating of the suitability or effectiveness of hindered phenols to hindered amines in the descriptions of WO 92/19993 and U.S. Pat. No. 5,283,101, no stabilizer of any kind is exemplified in the provided examples. Further, although the appliquxc3xa9 can employ polymers that are described as xe2x80x9chighly flexiblexe2x80x9d before and after electron-beam curing, neither the selected polymers nor the appliquxc3xa9 itself are described as xe2x80x9celasticxe2x80x9d. That is, a material can be highly flexible yet nonelastic as the terms xe2x80x9cnonelasticxe2x80x9d and xe2x80x9celasticxe2x80x9d are defined herein below. However, the reverse is not true; elastic materials are characterized as having a high degree of flexibility (i.e., Young""s Modulus of less than 10,000 psi (68.9 MPa) where lower modulus means more flexibility).
Although there is an abundance of art related to elastic ethylene polymer articles, including articles comprising curable, radiated and/or crosslinked ethylene polymers, and an abundance of art related to stabilized compositions and articles, there is no known disclosure of a polyolefinic elastic material with effective additive stabilization wherein the stabilization does not inhibit the desirable effects of irradiation and/or crosslinking (designed to impart elevated temperature elasticity and an increased melting point) and yet does inhibit the loss of elastic integrity (i.e. scission) when the material is subjected to a detergent washing and drying at elevated temperatures. Further, in another product brochure entitled, xe2x80x9cStabilization of Adhesives and Their Componentsxe2x80x9d, pp. 8-9 (1994), Ciba-Geigy, a premier stabilizer supplier, states that scission occurring in elastomeric materials (e.g. styrene-isoprene-styrene block copolymers) at elevated temperatures above 70xc2x0 C. is not readily controlled by the use of antioxidants.
As such, there is a present need for cost-effective, stable elastic articles having good elasticity at elevated temperatures as well as good washability and dryability. That is, there is a need for elastic articles which retain their shapes under strain at elevated temperature (for example, greater thanor equal to 125xc2x0 C.). There is also a need for a method of making elastic articles having good elasticity at elevated temperatures and good wash/dry stability. We have discovered that these and other objects can be completely met by the invention herein described.
We have discovered that elastic articles comprising curable, irradiated and/or crosslinkable ethylene interpolymers characterized by a polymer density of less than 0.90 g/cm3 at 23xc2x0 C. and at least one nitrogen-containing stabilizer exhibit excellent elasticity at room temperature and at elevated temperatures as well as excellent wash and dry stability. According to the broad aspect of the invention, there is provided a method of making a shaped curable, irradiated or crosslinkable article comprising at least one homogeneously branched ethylene interpolymer, which comprises ethylene interpolymerized with at least one other monomer and characterized as having (before being shaped, grafted, cured, irradiated, or crosslinked) a polymer density of less than 0.90 g/cm3 at 23xc2x0 C., and at least one nitrogen-containing stabilizer.
Another aspect of the invention is a method of making a shaped and cured, irradiated or crosslinked article comprising at least one homogeneously branched ethylene interpolymer, which comprises ethylene interpolymerized with at least one other monomer and characterized as having (before being shaped, grafted, cured, irradiated, or crosslinked) a polymer density of less than 0.90 g/cm3 at 23xc2x0 C., and at least one nitrogen-containing stabilizer.
A third aspect of the invention is a method of making an elastic article comprising the steps of:
(a) providing at least one homogeneously branched ethylene interpolymer having a density of less than 0.90 g/cm3 at 23xc2x0 C. having at least 0.1 weight percent of at least one nitrogen-containing stabilizer therein,
(b) fabricating or shaping the article from the interpolymer, and
(c) after the fabrication or shaping, subjecting the article to heat and/or ionizing radiation,
xe2x80x83wherein the article is characterized as having:
(i) a percent permanent set of less than or equal 25 at 23xc2x0 C. and 200 percent strain when measured at a 2 mil (102 mm) thickness using an Instron tensiometer after being shaped and cured, irradiated or crosslinked,
(ii) a percent stress relaxation of less than or equal 25 at 23xc2x0 C. and 200 percent strain when measured at a 2 mil thickness using a Instron tensiometer after being shaped and cured, irradiated or crosslinked, and
(iii) a percent stress relaxation of less than or equal 55 at 38xc2x0 C. and 200 percent strain when measured at a 2 mil thickness using an Instron tensiometer after.
A fourth aspect of the invention is a method of making an elastic article wherein the steps further comprises incorporating a pro-rad crosslink additive into the interpolymer.
A fifth aspect of the invention is a method of making a curable elastic article comprising the steps of:
(a) providing at least one homogeneously branched ethylene interpolymer characterized as having a density at 23xc2x0 C. less than 0.90 g/cm3 and comprising at least 0.1 weight percent of at least one nitrogen-containing stabilizer incorporated therein,
(b) preparing a melt of the stabilized interpolymer of (a);
(c) mixing into the melt of (b) from about 0.5 to about 5 phr of a silane crosslinker (parts of silane crosslinker per hundred parts interpolymer) while the crosslinker is at an ambient temperature between 0 and 30xc2x0 C.; and
(d) subjecting the melt mixture of (c) to ionizing energy or contacting the melt mixture of (c) with at least one free radical initiator to graft at least about 50 weight percent, based on the total weight of the crosslinker and the interpolymer, of the silane crosslinker to the stabilized interpolymer.
Preferably, the article is fabricated or shaped using an extrusion technique (i.e., the method consists of melting the interpolymer) such as, for example, a fiber melt spinning, fiber melt blowing, film blowing, cast film, injection molding, or rotomolding technique, and is permitted to cool or is quenched to ambient temperature (i.e., permitted to substantially solidify) before the application or exposures to (additional) heat, ionizing radiation and/or moisture.
In a preferred embodiment of the invention, the at least one homogeneously branched ethylene interpolymer is a substantially linear ethylene interpolymer. In another preferred embodiment, the ionizing radiation is provided by electron beam irradiation. In a third preferred embodiment, the at least one nitrogen-containing stabilizer is a hydroquinoline, diphenylamine or substituted piperidine.
We discovered that there is a subset of ethylene polymers which provide completely unexpected elastic performance results when cured, radiated and/or crosslinked. In particular, we found for a broad range of polymer densities, curing, radiation and/or crosslinking can dramatically decrease percent permanent set performance (i.e., improve elasticity or elastic recovery) and have no substantial-effects on ambient percent stress or load relaxation performance. However, while tending to adversely affect (i.e., increase) or have no affect on percent stress or load relaxation at elevated temperatures for polymer having densities equal to or greater than 0.865 g/cm3, surprisingly curing, radiation and crosslinking decreases (i.e., improves) the elevated temperature percent stress or load relaxation performance of ethylene interpolymer having a polymer density less than 0.865 g/cm3 or a DSC crystallinity at 23xc2x0 C. less than 8.5 weight percent. That is, curing, radiating and/or crosslinking is an effective means for providing elastic materials and articles characterized as having excellent elevated temperature stress relaxation characteristics.
Not only is the dramatically different response to irradiation or crosslinking surprisingly in itself, these results are surprising for another reasons as well. For example, these results are surprisingly and unexpected because at a density less than 0.90 g/cm3, ethylene interpolymers are already substantially amorphous. That is, a cross-over or transition in elastic performance attributable to curing, radiation and/or crosslinking would ordinarily be expected to relate to the amorphosity of the polymer; however, according to hexane extraction data at 50xc2x0 C., determined according to the Food and Drug Administration (FDA) test method set forth under 21 37 C.F.R. xc2xa7xc2xa7177.1520 (d)(3)(ii), ethylene polymers are substantially amorphous at a density of 0.89 g/cm3 and below. Given such small differences in amorphosity or crystallinity, dramatic elasticity differences in response to irradiation or crosslinking simply would not ordinarily be expected.
As another surprise, we discovered that the incorporation of at least one nitrogen-containing stabilizer imparts excellent laundering characteristics to the elastic article. This discovery is surprising and unexpected because the stabilizer does not inhibit or interfere with effective curing, radiation effects, crosslinking or crosslinking effects (and as such permits substantial melting point increases, i.e., from less than 75xc2x0 C. to greater than 125xc2x0 C.), yet inhibits melting and flowing (i.e., scission) from occurring at substantially elevated temperatures (e.g., 133xc2x0 C.) in a wash/extend dry testing.
The washing and drying performance results of the inventive article are also surprising for at least one other reason. That is, the effectiveness of the at least one nitrogen-containing stabilizer is unexpected because in ordinary stabilization tests (e.g., inhibition of yellowing) nitrogen-containing stabilizers perform comparable to phenolic stabilizers, yet phenolic stabilizers do not inhibit melting and flowing in wash/dry testing.